I confess that I hadn't seen that black is black article. Again, as per the comments on that documentation thread, the information is all out there, but finding it is a challenge and an archaeological dig. I just subscribed to that blog so as not to miss any more of these.
That info is what I'm looking for. However I hesitate to say this, given how little I know, but I don't believe the WN Opaque 1 number of 1.409 on HPR. Are you sure about this? I just did this test, and after a night of drying my measurement is now at 1.660. And that's without making any allowance for density drop-off from overloading, as pointed out elsewhere by Richard Boutwell. My number is broadly consistent with what Tyler and Mark Sonners reported, but yours is not.
Are you really saying that there's such a difference between this ink and the other shade 1's? What would cause this? Based on my measurement, WN Opaque 1 is pretty much exactly the same as Selenium 1 (your measurement of 1.661), and as WN 1 is better on gloss I think it's the clear choice.
Yes, I understand that in practice printing 100% black using a piezo curve will combine quite a few shades. On my printer it uses four with perhaps a touch of shade 5. I understand that not much of shade 1 is used, so its impact is minor. I understand that piezography printing is all about open shadows and shadow detail. And retaining shadow detail on gloss can be a challenge, given its greater dmax. But if we're going to go to the trouble and (minor) expense of switching inks then I'd like to know that there's some (minor) benefit, and right now I can't see it.
I personally don't think that a universal black is confusing, and I see a marketing opportunity.